1. Field of the Invention
The present invention relates to a method for making a load resistor on a semiconductor chip.
2. Description of the Prior Art
Resistors with high resistance on a semiconductor chip are usually made of polysilicon, such as resistors made in static random access memory (SRAM) . Resistors made of polysilicon are used to replace load transistors in SRAM in order to reduce the number of transistors. Such method can save production costs and maximize circuit integration.
Please refer to FIGS. 1 and 2. FIGS. 1 and 2 show a prior art method for making a load resistor on a semiconductor chip 10. The surface of the semiconductor chip 10 comprises a Si substrate 12, a neutral silicate glass (NSG) layer 14 deposited on the Si substrate 12 for functioning as an insulation layer and a line-shaped polysilicon layer 16 deposited on the NSG layer 14 used as a conductive path. As shown in FIG. 1, ion implantation of As ions is performed to lightly dope the polysilicon layer 16 on the surface of the semiconductor chip 10 to eliminate defects and to average out the resistance value of the polysilicon layer 16.
Please refer to FIG. 2. By using photolithographic and etching techniques, an optical resistance 18 is formed on the surface of the polysilicon layer 16 for acting as a mask or sacrificial layer in the ion implantation process. In the ion implantation process, the polysilicon layer 16 of the surface of the semiconductor chip 10 is doped with P ions to reduce the resistance value of the entire polysilicon layer 16 and to make the polysilicon layer 16 an electric conductor. The area 20 of the polysilicon that is covered by the optical resistance 18 forms a load resistor with relatively high resistance.
However, in order to achieve high resistance (about a few hundred G.OMEGA.) in an integrated circuit, the deposited polysilicon must be very large. If the shallow trenches measure less than 0.25 .mu.m in width, the load resistor may become too large and will occupy a lot of space.